This invention relates to method of measuring internal data of an object using Fourier transform nuclear magnetic resonance, and more particularly to a method of suppressing signals of a solvent having a large signal quantity which will otherwise impede measurements of signals of low concentration solutes in the solvent such as lactic acid, amino acid, etc., in a living body.
In a system wherein low concentration solutes and a solvent co-exist, the peaks of nuclear magnetic resonance signals of low concentration solutes are hidden at the skirt of the large peak of the solvent molecule and for this reason, the signals of the solvent molecule must be suppressed. This signal suppression can be achieved by a method which saturates nuclear magnetizations of the solvent molecule by utilizing the difference of chemical shifts and a method which observes selectively the signals of multiple quantum transition and suppresses single quantum transition of the solvent molecule. Of these, the method which has gained the widest application in the past is the one that applies a plurality of radio frequency pulses the degree of flip angles of which becomes a ratio of binominal coefficients, with suitable pulse intervals by utilizing the phase difference of the solvent molecule and the low concentration solute molecules after excitation, and excites only the nuclear magnetizations of the low concentration solute molecules. The method of saturating the nuclear magnetization of the solvent molecule by utilizing the difference of the chemical shifts is described in R. L. Vold et al "Measurement of Spin Relaxation in Complex Systems" (J. Chem. Phys., 48, pp. 3831 (1968), and the method of suppressing the single quantum transition of the solvent molecule by observing selectively the signals of the multiple quantum transition is described in C. L. Dumoulin "A multiple Quantum Filter for the Imaging of Homonuclear Spin-Spin Coupled Nuclei", (Proceedings of the Fourth Annual Metting of the Society of Magnetic Resonance in Medicine, London, pp. 145 (1985).
In accordance with the method which applies a plurality of radio frequency pulses the degree of flip angles of which becomes a ratio of binominal coefficients, with suitable pulse intervals by utilizing the phase difference of the solvent molecule and the low concentration solutes after excitation and excites only the nuclear magnetizations of the low concentration solute molecules, pulse sequences can exist infinitely in accordance with the number of terms of binominal coefficients. FIG. 2 shows a measuring method using a pulse sequence comprising radio frequency pulses having a flip angle ratio of 1:-3:3:-1, which is described in "Journal of Magnetic Resonance", 55, pp. 283-300 (1983), by way of example.
If the effect of relaxation is neglected, the observation signal becomes maximal when the sum of the effective flip angles is .pi./2. In this case, therefore, the flip angle of the first pulse `1` in the pulse sequence described above is .pi./16. When the measuring method described above is employed under this condition, the trajectory of the nuclear magnetization vector in a rotating coordinates system which rotates with a frequency equal to the frequency of the radio frequency pulses is shown in FIG. 3A and the trajectory of the nuclear magnetization of the low concentration solutes in which the sum of the effective flip angles becomes .pi./2 is shown in FIG. 3B. Here, the direction of the static magnetic field is a z direction and the phases of the radio frequency pulses are those which rotate the magnetization with the x axis of the rotating coordinates system being its axis of rotation. The frequency of the radio frequency pulses is equal to the resonance frequency of the solvent molecule.
In FIG. 2, the nuclear magnetizations of both solvent molecule and low concentration solutes exist inside a y - z plane immediately after the application of the pulse `1` and the angle between the z axis and the nuclear magnetizations is .pi./16. The pulses are spaced a part from one another with a pulse interval time .tau. and during this time .tau., a phase difference in accordance with the frequency difference of the precession occurs between the nuclear magnetization of the solvent molecule and that of the low concentration solutes. In the nuclear magnetizations shown in FIG. 3B, this phase difference is .pi.. The nuclear magnetization of the solvent molecule receives a flip of -3.pi./16 immediately after the application of the pulse `-3`, so that the angle between the z axis and the nuclear magnetization of the solvent molecule is -2.pi./16. On the other hand, since the net flip the nuclear magnetizations of the low concentration solutes receives by the pulse `-3` is 3.pi./16, the angle between the z axis and the nuclear magnetizations of the low concentration solutes is 4.pi./16.
Next, when a pulse `3` is applied after the passage of the pulse interval time .tau., the angle between the z axis and the nuclear magnetization of the solvent molecule is .pi./16 and the angle between the z axis and the nuclear magnetizations of the low concentration solutes is 7.pi./16. Furthermore, when a pulse `-1` is applied after the pulse interval time .tau., the angle between the z axis and the nuclear magnetization of the solvent molecule becomes finally zero and the solvent signal is no longer observed. On the other hand, the angle between the z axis and the nuclear magnetizations of the low concentration solutes is 8.pi./16=.pi./2 and the observation signal becomes maximal.
This measuring method has the advantages that the suppression effect of the solvent signal is relatively high and that when a pulse sequence comprising even-numbered radio frequency pulses is used, the suppression effect of the solvent signal is not adversely affected even when inhomogeneity of the radio frequency magnetic field exists.